1. Field of the Invention
The present invention relates to a crank angle sensor which detects the angular position of the crankshaft of an internal combustion engine, and particularly to one in which magnetoresistive elements are used.
2. Description of the Related Art
Crank angle sensors using magnetoresistive elements are disclosed in Japanese Patent Laid-Open No. 57-133311. The crank angle sensor disclosed therein uses, for example, 3 disks of the same diameter, the peripheries of which are circumferentially divided into n, n.times.2.sup.1 and n.times.2.sup.2 sections respectively, n corresponding to the number of cylinders of an engine. Each section thus divided is provided selectively with either one of magnetic polarities N and S. Those disks are coaxially stacked together to form a cylindrical rotor of the sensor. As a result, codes consisting of 3 bits arranged in the axial direction are circumferentially distributed over the entire surface of the cylindrical rotor. Each code is formed by magnetic polarities N and S which are made to correspond to a logical "1" and "0", respectively. A stator of the sensor made of a magnetoresistive element faces the cylindrical rotor so as to detect the magnetic polarities provided on the circumferential surface of each disk. Since the rotor of the sensor is rotated in synchronism with the rotation of a crankshaft of the engine, the code detected by the sensor indicates the angular position of the crankshaft.
However, the apparatus as described above has the following problem. Mutual magnetic interference occurs between the disks of the rotor, since the disks have different N-S pitches and are stacked close together. Moreover, the difference in the N-S pitch between disks is large. Further, there is another problem in that signal edges are not well defined and accuracy is reduced as the pitch of the magnetic pole of a section increases. Although the edge of a signal becomes well defined and accuracy is excellent when the pitch of the magnetic pole of a section is shortened, the shorter pitch of a magnetic pole increases the mutual magnetic intereference.
Further, a crank angle sensor for use in engine control must be able to detect not only the angular position of the crankshaft, but also other signals, such as a reference signal indicating a reference position of the crankshaft, or a cylinder signal for each respective cylinder. To this end, a detector of the type disclosed in Japanese Utility Model Laid-Open Application No. 55-179317, for example has been proposed. According thereto, information representative of the angular position and the reference position of the crankshaft is provided by magnetic poles arranged separately in two rows on the circumferential surface of a cylindrical rotor. Also in such an apparatus, there occurs the same problem as occurs in the sensor described in the former application, since the N-S magnetic pole pitch for deriving reference position information is different from that for angular position information to a large extent. If the magnetic pole pitch for each of the two types of information is the same, another problem will occur in that the output pulse width of the two types of information will be equal. It is necessary for effecting engine control that the pulse width of the angular position information be several times narrower than that of the reference position information.